(1) Field of the Invention
This invention relates to an optical waveguide or optical fiber, and more particularly to an optical transmission line which allows the transmission of only limited modes for use in an optical communication system.
(2) Description of the Prior Art
As the optical waveguide, the so-called optical fiber employing glass is generally known. In recent years, it has rapidly progressed to render the transmission loss of the optical fiber low, and it has become possible to obtain an optical fiber which can transmit light in the wavelength region of a semiconductor light source at a loss lower than several decibels per kilometer. This signifies that the transmission length or the repeater spacing can be extended in the optical communication system which utilizes the optical fiber as the waveguide or transmission line. On the other hand, however, the optical fiber is subject to the fundamental restriction that the product between the transmission bandwidth and the transmission length (hereinafter, termed the BL product) is constant. Therefore, when the transmission length is stretched, the transmission band cannot avoid becoming narrow accordingly. In order to fully exploit the low loss property of the fiber, therefore, the BL product of the optical fiber need be made large.
The first method for making the BL product of the optical fiber large is to render the propagation mode of the optical fiber single, i. e., to put the optical fiber into the single mode fiber. In general, the fiber is constructed of a core of comparatively high refractive index and a cladding layer, and the diameter of the core is as very small as several microns. With this method, therefore, it is difficult to launch light into the optical fiber and to connect the optical fibers to each other.
The second method is to form the so-called graded-core fiber in which the refractive index distribution in the section of the optical fiber is continuously lowered from the center towards the periphery. Ideally, the refractive index distribution need be a parabolic distribution. However, insofar as the optical fiber is finite in the radial direction, the parabolic distribution of refractive indexes inevitably falls into a shape in which the parabola is cut off halfway. In this case, the group delay of propagation modes near the cut-off point becomes smaller than that of the other modes, which becomes a factor for the limitation of the band.
Among the modes propagating over the optical fiber, there are the skew mode in which a light beam travels helically with respect to the lengthwise direction of the optical fiber and the meridional mode in which a light beam meanders on a certain fixed vertical section. Any refractive index distribution which can make zero the group delay difference among all the modes including such modes cannot exist. In this sense, whatever refractive index distributions are given, the graded-core optical fiber cannot bestow a perfect focussing action on all the modes. Accordingly, as the light transmission length becomes larger, the focussing action becomes weaker. As the result, the radial distribution of the propagation mode fields (or the mode volume) of the optical fiber spreads and finally arrives at the discontinuous boundary of the refractive index distribution. At last, all the propagation modes come to undergo the total reflection at the boundary. Under such state, the transmission characteristic of the graded-core optical fiber becomes similar to that of the cladding type of the two-layer structure, and the effect of widening the band owing to the continuous variation of the refractive index distribution cannot be achieved.
Further, the third method forms a three-layer structure (W-type optical fiber) which consists of a core having the highest refractive index, an intermediate layer having the lowest refractive index, and a cladding having a medium refractive index (U.S. Pat. No. 3,785,718). According to this method, the components of higher order modes are made leaky and the components of propagation modes are limited to only lower order modes. Thus, the group delay difference is made small, and the transmission bandwidth is expanded.
In order to expand the transmission bandwidth by the optical fiber, however, the difference between the refractive indexes of the core and the cladding layer need be made very small. By way of example, in order to make the BL product greater than 200 MHz.km, the refractive index difference need be made below 0.05%. To this end, the required accuracy of refractive indexes in the process of manufacturing the optical fiber is at least 0.01% or so. This is extremely difficult technically.
As the forth method, there has been proposed an optical fiber composed of a central layer whose refractive index decreases gradually from the center in the radial direction, and a layer which is provided around the central layer and which has a refractive index being lower than the highest refractive index of the central layer and being higher than the lowest refractive index thereof.
According to the optical fiber, mode components whose mode index (the ratio of the phase constant to the wave number) lies between the refractive index of the central layer and that of the cladding layer are propagative and do not suffer from any loss. In contrast, mode components whose mode index lies between the refractive index of the cladding layer and the lowest refractive index are coupled with the cladding layer and consequently become leaky, and suffer from reactive losses. Since such leaky modes have greater group delays than the propagation modes, their existence gives rise to lowering in the transmission bandwidth, and they are unnecessary for rendering the band wide. A sufficient attenuation need therefore be bestowed on the leaky modes. However, once the refractive index distribution of the central layer has been determined, the radial width of a part whose refractive index is lower than that of the cladding layer is uniquely decided. It is accordingly impossible to freely control the attenuation of the higher order modes being the leaky modes, and the structure and the design lack in versatility. Further, after a long distance transmission, a portion in which the propagation modes exist becomes, at last, that part of the central layer at which the mode index is higher than the refractive index of the cladding. Therefore, the diameter of the substantial core (central layer) is considerably smaller than the inside diameter of the cladding layer. This is inconvenient in launching light into the optical fiber and in optically connecting the optical fibers to each other.